1. Field of the Invention
The present invention relates generally to a cushioning device for even distribution of a blank-holding force to a blank to be processed on a press. More particularly, the present invention is concerned with a method and an apparatus that permits easy and accurate diagnosis on a range of the blank-holding force within which the blank-holding force is substantially evenly distributed on the blank.
2. Discussion of the Related Art
A press has a slide with an upper die attached thereto, which is lowered toward a lower die to perform a pressing operation on a blank or workpiece while the blank is held by and between the upper die and a pressure member. For holding the blank during a pressing cycle, there is known a cushioning device which includes (a) a cushion platen or pad which receives a blank-holding force (cushioning force) produced by suitable force generating means, (b) a plurality of balancing hydraulic cylinders disposed on the cushion platen and having respective fluid chambers which communicate with each other, and (c) a plurality of cushion pins linked at their lower ends with the respective hydraulic cylinders and supporting at their upper ends the pressure member, so that the blank-holding force produced by the force generating means is applied to the pressure member through the cushion platen, hydraulic cylinders and cushion pins. The mutually communicating hydraulic cylinders function to assure substantially even distribution of the blank-holding force on the cushion pins, that is, substantially even distribution of the blank-holding force on the pressure member.
An example of such cushioning device is disclosed in laid-open Publication No. 1-60721 (published in 1989) of unexamined Japanese Utility Model Application. This cushioning device is adapted to apply the blank-holding force to the pressure member such that the blank-holding force acts on the pressure member substantially evenly over the entire surface area of the pressure member to thereby assure substantially uniform distribution of the surface pressure of the pressure member with respect to the blank, for permitting pressing cycles to be performed with high stability of accuracy, irrespective of a length variation or difference of the cushion pins, tilting of the cushion platen with respect to the nominal plane, and other undesirable fluctuating factors of the cushioning device.
For substantially even distribution of the blank-holding force on the pressure member, it is required that the pistons of all the balancing hydraulic cylinders of the cushioning device be positioned between their upper and lower stroke ends, that is, placed at their neutral position during a pressing cycle, even in the presence of fluctuating factors such as the length variation of the cushion pins. To this end, an optimum initial hydraulic pressure Pso to be applied to the hydraulic cylinders prior to a pressing operation to establish the desired even distribution of the blank-holding force on the pressure member is determined so as to satisfy the following equation (1): EQU Xav=(Fs-n.multidot.As.multidot.Pso)V/n.sup.2 .multidot.As.sup.2 .multidot.K(1)
where,
Xav: average operating stroke of the pistons of the hydraulic cylinders (cushion pins), PA1 As: pressure-receiving area of the piston of each hydraulic cylinder, PA1 K: volume modulas of elasticity of the working fluid, PA1 V: total fluid volume in the hydraulic cylinders and the hydraulic circuit connected thereto, PA1 Fs: blank-holding force, PA1 n: number of the hydraulic cylinders (cushion pins). PA1 Fs: blank-holding force acting on the pressure member, PA1 Wr: weight of the pressure member, PA1 As: pressure-receiving area of each balancing hydraulic cylinder, PA1 Psx: hydraulic pressure in the hydraulic cylinders, PA1 Wp: average weight of the cushion pins, PA1 n: number of the cushion pins (number of hydraulic cylinders linked with the cushion pins). PA1 Aa: pressure-receiving area of the pneumatic cylinder, PA1 Wa: total weight of the cushion platen and hydraulic cylinders.
The average operating stroke Xav of the pistons of the balancing hydraulic cylinders is predetermined by experiments, for example, so as to enable all the cushion pins to abut at their upper ends on the pressure member while the pistons of the hydraulic cylinders are positioned away from their upper stroke ends by the cushion pins, but do not reach their lower stroke ends due to collision of the upper die with the pressure member through the blank during a pressing action on the blank, even if the cushion pins have different length dimensions and/or the cushion platen is tilted some angle with respect to the nominal horizontal plane. The total fluid volume V is a total volume of the working fluid which fills the fluid chambers of all the hydraulic cylinders when the pistons are located at their upper stroke ends, plus a volume of the fluid which fills the hydraulic circuit connected to the hydraulic cylinders.
For accurate calculation of the optimum initial hydraulic pressure Pso, it is required that the average operating stroke Xav, pressure-receiving area As, volume modulas of elasticity K and total fluid volume V used to calculate the optimum initial hydraulic pressure Pso be determined as precise as possible. In this sense, these values should not be theoretically calculated values but should rather be obtained by experiments or tests performed on the individual pressing machines which have specific operating characteristics. These experiments are extremely cumbersome and time-consuming. Yet, the values obtained by the cumbersome experiments may include some errors, which lead to errors in the calculated optimum initial hydraulic pressure Pso, resulting in the failure to establish even distribution of the blank-holding force Fs on the pressure member for even distribution of the blank-holding surface pressure, if the hydraulic pressure of the hydraulic cylinders is adjusted according to the calculated optimum initial pressure value Pso. Thus, the product obtained from the blank may be defective.
Once the optimum initial hydraulic pressure Pso of the balancing hydraulic cylinders is determined as described above, the blank-holding force Fs is almost evenly distributed on the pressure member even if the blank-holding force Fs is changed to some extent. However, almost even distribution of the blank-holding force Fs may be lost when the blank-holding force Fs is adjusted to an optimum level for a specific die set by using a try press, or when the force Fs is adjusted on a pressing line for some reason or other. This drawback may occur since the operator who adjusts the blank-holding force Fs does not know the range of the force Fs within which the force Fs can be almost evenly distributed on the pressure member. Although the even distribution of the blank-holding force Fs can be maintained if the optimum initial hydraulic pressure Pso is adjusted according to the above equation (1) each time the blank-holding force Fs is adjusted, this procedure upon test operation on the try press or upon adjustment of the force Fs on the production line is cumbersome and leads to low production efficiency.